In the manufacture of semiconductor integrated circuits (ICs), semiconductor IC die (or chips) are commonly mounted on a leadframe, followed by enclosing the IC die and part of the leadframe in a plastic casing to form an IC package. The IC package can be mounted on a printed circuit board (PCB) for interconnection with external circuitry. A leadframe should provide good bondability, molding compound characteristic, and solderability, so that it can facilitate the packaging process. To provide these characteristics, various coatings may be formed on the leadframe surface, which typically comprises a copper or a copper alloy base material.
A conventional method for providing improved bondability for the interconnection between bond wires and the bonding areas of a leadframe is to electroplate a metal such as silver (Ag) on the bonded areas including on the surface of the metal terminals (e.g., copper terminals) within the package before wire bonding. It is also known to form a sintered metal material on the bonded areas of the metal terminals by sintering a deposited metal paste before wire bonding.
Wire bonding is then generally performed by a first bonding which forms a ball bond by placing a capillary over the bond pad (or die pad) of the IC die with a ball of the wire extending out of the capillary, and then a second bonding for bonding to the metal terminal. In the second bonding the capillary may be moved to a metal terminal (e.g., lead finger which as noted above can include a plated metal layer or a sintered metal material thereon) of the leadframe to which a second bond is made with the wire travelling with respect to the capillary bore, and a stitch bond can be made to the metal terminal (e.g., lead finger) using the capillary with the wire then being broken, leaving a small wire pigtail extending out of the capillary.
Wire bonding can also be used to bond a semiconductor die to a variety of package substrates other than leadframes. For example, package substrates can include printed circuit boards (PCBs) including multi-layer PCBs, thick film ceramics, glass substrates and flexible circuits.
Metal pastes are conventionally sintered in a cure oven to remove the binder (and solvent if present) and to densify (reducing the porosity) of the metal material. One known process for producing a sintered metal material comprises providing a copper paste including a binder and a plurality of copper nanoparticles, and then heat treating the metal paste in a non-oxidizing atmosphere, such in N2 or an inert gas at a peak temperature of about 150° C. to 300° C. to remove the binder.
In the case of wire bonded devices having wire bonds to a sintered metal material on a metal terminal of a package substrate, however, there can be a problem with weak wire bond connections and resulting instability of the wire bonds to the metal terminals. Weak wire bond wire connections can cause high resistance contacts and low pull strength of the wire bond leading to pulling apart. Conventional solutions to this problem include modifying the metal paste formulation (composition), applying a radio frequency (RF) plasma to the metal paste in the sintering cure oven, and applying mechanical pressure to the metal paste, which can each reduce porosity and thus increase the density of the sintered metal material by a modest amount.